Fair Value of Financial Instruments Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of policy loans	$ 3,133.8	$ 3,051.4
Embedded Derivative, Gain (Loss) on Embedded Derivative, Net	51.8	(39.4)	21.1
Other Than Fair Value Inputs Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available For Sale Securities Percent Of Total Fair Value	95.10%
Fair Value, Inputs, Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Description	1212.0
Fair Value, Inputs, Level 1 [Member] | Pricing Service [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available For Sale Securities Percent Of Total Fair Value	4.90%
Fair Value, Inputs, Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Long-term Debt, Description	1756.8
Fair Value, Inputs, Level 2 [Member] | Pricing Service [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available For Sale Securities Percent Of Total Fair Value	77.50%
Fair Value, Inputs, Level 2 [Member] | Other Observable Market Data
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available For Sale Securities Percent Of Total Fair Value	3.80%
Fair Value Inputs Level 2 Or Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available For Sale Securities Percent Of Total Fair Value	13.80%
Ceded Policy Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying amount of policy loans	$ 2,912.7	$ 2,838.3